The use of aluminum alloys in the manufacture of automobile bodies and components has increased in part due to the need to reduce the weight of the vehicles for improved fuel economy. One application for aluminum alloys in the manufacture of automobiles is in the forming of body panels from aluminum alloy sheet stock. For example, hoods, doors and deck lids are formed by stamping an inner panel and an outer panel from suitable aluminum sheet stock. The outer panel forms the decorative and functional outline of the vehicle panel. The inner panel serves a reinforcing function. In the manufacture, then, of such panels and the like, it is often necessary to trim or pierce a previously stamped aluminum sheet. However, aluminum alloys have different forming characteristics than the long-used low carbon steel sheets. For example, the trimming or piercing of such steel sheets rarely yields metal slivers that mar the part or the trim or piercing dies.
A series of aluminum sheet alloys have been developed which are strong and hard due to the presence of precipitated, finely divided hardening particles. One such series is the AA2XXX series in which small amounts of copper and magnesium, for example, are added to the aluminum alloy to contribute to hardening particle formation. Another series is the AA6XXX series where silicon, magnesium and copper are added for hardening. A third series is the AA7XXX series where zinc, magnesium and copper, for example, are added as hardening constituents. These alloys are well known and commercially available. They are formed into sheet stock from cast billets by a suitable sequence of hot rolling and cold rolling operations. Usually at the finish of the sheet forming/rolling operation, the sheet material is heated to dissolve in solid solution the small amounts of prospective hardening particles or transition phases such as Mg.sub.2 Si or GP zones (e.g., in the 6XXX series) and the like. The sheet is then quenched to retain the secondary phases in an unstable solution. The quenched material may be allowed to age at room temperature, whereupon the dissolved hardening constituents slowly reprecipitate in a very finely divided state to strengthen and harden the sheet. Such room temperature-aged alloys are usually identified as having a T4 temper designation. In some cases, the alloy is heated to a temperature of between 150.degree. C. and 200.degree. C. after the quenching operation to induce reprecipitation of the hardening phases. The alloy is then designated as being in a T6 temper condition. The T6 alloys are usually stronger and harder than the T4 alloys. The terms "age hardening" and "precipitation hardening" are used interchangeably herein to include aluminum alloys aged at room temperature and alloys heated above room temperature to accelerate or increase the strengthening and hardening effect.
Thus, when an automobile body panel is formed from an aluminum alloy such as AA6111-T4, it is in an age-hardened condition. The properties of the alloy are a compromise which enable it to undergo suitable stamping, drawing, trimming and piercing operations and the like for shaping into a body panel and yet provide suitable strength and hardness in the finished panel.
A difficulty is that such age-hardened alloys, for example, the AA 2XXX, 6XXX and 7XXX series, are not as ductile as low carbon steels used for automobile body panels, and the aluminum alloys produce slivers when sheared as in a trimming or piercing operation. Sliver formation in trimming and piercing operations is a major problem in manufacturing of aluminum sheet metal parts. Metal slivers are produced due to burnishing and fracturing of the aluminum at the edge of the trimmed or pierced sheet. These slivers are transferred to the sheet or tools during subsequent operations and produce scratches or other defects in formed panels. Hand finishing of formed parts is often required to eliminate the defects produced by the slivers and give an acceptable class A finish. Metal slivers are a larger problem with aluminum stampings than with steel for two reasons: (1) aluminum fractures differently than steel, leading to a larger number of slivers at the sheared edge, and (2) aluminum is softer than steel and more susceptible to damage from the slivers during handling between forming operations.
Accordingly, it is desirable to have a practice for performing die shearing operations on a sheet of age-hardened aluminum alloys without producing slivers.